Gas detection instruments are used to separate chemical ions from a gas and detect the ions. Existing instruments, however, all have drawbacks that make them expensive to build, burdensome to operate, and difficult or impossible to miniaturize. An Ion Mobility Spectrometer (IMS) is a gas detection instrument in which gas ions are separated according to their individual velocities as they drift through an electric field. Most traditional large non-portable IMS systems use electro-spray ionization to ionize chemical molecules, but this ionization source is too complex to be cost-effectively down-sized and integrated with other components. Other ionization techniques such as surface ionization have been used, but most of the ionization techniques require a high-vacuum environment for input sample gas, which is very challenging to be implemented in a miniature IMS system. As a result, a new ionization source that can be operated in atmospheric pressure with scalable size is necessary.
A small-scale IMS device has been reported by Sandia National Lab, but this miniature IMS drifter involves too many parts and electrical connections, which results in much lower device fabrication throughput and much higher package and assembly cost. Moreover, this IMS has a measured dimension in the range of 10 cm×2 cm×2 cm, but still needs further size reduction before it can be used as a fully-assembled handheld gas detection system.
Draper Labs developed a miniaturized Radio Frequency-IMS (rf-IMS). This rf-IMS has significant drift channel size reduction due to the simplification of the drift or separation electrodes by using High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) technology to filter ions in the drift channel. Such FAIMS technology requires a very high radio frequency (RF) electric field to filter the ions in the drift channel, with a voltage of 1700V at 2 MHz frequency. The corresponding high-voltage RF power supply consumes very high power and also requires special microwave protection. Meanwhile, the electronics for producing such high voltage RF signal are very expensive and usually very large, which in turns leads to difficulty in producing a low-cost miniature gas detector system.
In both current state of the art miniature gas detection systems, radioactive materials were used as the ionization source in order to keep small system footprint: the Sandia IMS uses radioactive 241 Am as its ionization source to reduce the system size, while the Draper Labs rf-IMS uses radioactive 63Ni as the ionization source. The use of radioactive materials raises its own problems. Regular leak tests must be performed to work with such materials. Meanwhile, special safety regulations and licensing requirements can limit the commercial acceptance of devices using radioactive material. Radioactive waste disposal also raises serious concerns about environmental impacts. Therefore, the development of an ambient pressure ionization source that can replace radioactive material is desired.
Current miniature gas detectors are still constructed by separate individual components—separate ionization sources, separate ion drift and separation channels, and separate ion detector—which require significant amount of assembly efforts and thus higher cost. These separate components cannot be monolithically integrated in fabrication and require significant efforts on the assembly, which increases the device cost. A more robust miniaturized gas detector that can be inherently integrated for low-cost mass production is desirable.